The present invention relates to an optical head for use in an optical disk apparatus for recording, reproducing and erasing information on an optical disk by using a laser beam, and more particularly, to a multi-beam split-type optical head which has an optical system split or divided into a stationary section and a movable section, and which can irradiate an optical disk medium simultaneously with a plurality of laser beams to allow recording and reproduction to be simultaneously performed on and from different tracks.
Conventionally, there has been known a multi-beam optical head which simultaneously irradiates an optical disk medium with a plurality of laser beams to reproduce recorded information simultaneously or to monitor such recorded information immediately after the recording, or to simultaneously record or reproduce information in parallel on a plurality of different tracks. The structure of this type of optical head has an optical system split into a stationary section and a movable section, i.e., a split-type optical system, for example, as described in an article entitled "A New Composite Tracking Method for Two-beam Split-type Optical Head" from an optical memory symposium held at Makuhari Messe on Jul. 9, 1990. Specifically, two laser beams having different wavelengths from each other are overlapped by a dichroic mirror in the stationary section, and these overlapped laser beams are introduced to a single objective lens common to both beams disposed in the movable section to form a plurality of light spots at predetermined intervals on an optical disk medium.
When information recorded on an optical disk medium by the plurality of light spots is to be reproduced for monitoring immediately after the recording, these light spots are placed on a single track as front and rear spots with slight distances therebetween. On the other hand, when information is to be simultaneously recorded or reproduced on a disk medium in parallel by the plurality of light spots, these light spots are placed on a plurality of adjacent tracks on the disk with a slight distance therebetween.